1. Field of the Invention
The present invention is in the field of notch or band stop filters, and it relates particularly to such notch filters which may be employed in connection with audio systems to eliminate or reduce 60 Hz "hum" and sometimes also harmonics thereof.
2. Description of the Prior Art
60 Hz "hum" problems are common in audio equipment, and are particularly troublesome in television and radio broadcasting, recording studios and commercial sound systems where the audio equipment is of very high quality and its sensitivity tends to make it responsive to spurious signals. This problem is frequently caused by low level audio program cables which may lie in close proximity to high current power line cables such as, for example, big 100 to 300 amp studio lighting cables. In many cases, this proximity between audio program cables and power line cables, and consequent crossover of 60 Hz "hum" and some harmonics into audio cables, is difficult to avoid and would consume expensive time in attempts to locate the source of the problem and correct the problem by mechanical separation of cables.
A simple answer to this problem would be the insertion of a notch filter tuned to 60 Hz in any audio line which may be vulnerable to 60 Hz "hum", and also to insert one or more notch filters tuned to harmonics of 60 Hz as may be required in vulnerable audio lines. However, prior to the present invention there has been no line notch or band stop filter available that had a sufficiently narrow stop band to avoid causing a large and intolerable hole in the low frequency response, or in the case of harmonics to avoid causing a large gap in the midrange response. Also, the typical state-of-the-art notch or band stop filter did not provide sufficient attenuation to completely remove 60 Hz "hum" and its harmonics from high quality audio equipment.
Thus, the typical prior art line notch filter had a stop bandwidth of about 30% of the stop frequency, with a response curve slope no steeper than about 100 db per octave and an attenuation of about 30 db. The best prior art notch filters still had a bandwidth of at least about 25% of the stop frequency and a response curve slope up to about 150 db per octave for active notch filters and up to not more than about 500 db per octave for passive notch filters, with an attenuation of as much as about 50 db; however such notch filters were very bulky or involved extensive circuitry, and they were expensive. The bandwidth of a notch filter is considered in the art to be the bandwidth between the "corner frequencies" on the response curve below and above the stop frequency at the points where the filter produces an attenuation of 3 db. The prior art line notch filter attenuation bandwidth of approximately 25%-30% of the stop frequency results in a bandwidth of approximately 15-18 Hz for the 60 Hz "hum" stop frequency, and the resulting 15-18 Hz low frequency hole in the response of the system causes such prior art notch filters to be totally unacceptable for high quality audio systems. Often, 2nd, 3rd and 4th harmonics are involved in the 60 Hz "hum" problem, and if such a prior art notch filter with a 25%-30% bandwidth were tuned to the 4th harmonic, which is 240 Hz, a wide band of 60-72 Hz would be essentially removed from the middle of the midrange, which again would be totally unacceptable.
It is recognized that some hybrid circuits have been developed for telephone equipment which provide attenuation slopes up to approximately 300 db per octave, however, such hybrids are not adaptable to the notch filter situation, as they are either low pass or high pass filters only, and they are very expensive.